1. Field of the Invention
The present invention relates to estimation of amplitude and phase imbalance in wireless direction conversion receivers, for example an IEEE 802.11a based Orthogonal Frequency Division Multiplexing (OFDM) receiver.
2. Background Art
Local area networks historically have used a network cable or other media to link stations on a network. Newer wireless technologies are being developed to utilize OFDM modulation techniques for wireless local area networking applications, including wireless LANs (i.e., wireless infrastructures having fixed access points), mobile ad hoc networks, etc. In particular, the IEEE Standard 802.11a, entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band”, specifies an OFDM PHY for a wireless LAN with data payload communication capabilities of up to 54 Mbps. The IEEE 802.11a Standard specifies a PHY system that uses fifty-two (52) subcarrier frequencies that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM), or 64-QAM.
Hence, the IEEE Standard 802.11a specifies an OFDM PHY that provides high speed wireless data transmission with multiple techniques for minimizing data errors.
A particular concern in implementing an IEEE 802.11 based OFDM PHY in hardware involves providing a cost-effective, compact device the can be implemented in smaller wireless devices. Hence, implementation concerns typically involve cost, device size, and device complexity.
FIG. 1 is a diagram of a typical direct conversion receiver. The direct conversion receiver 10 includes an antenna 12, a low noise amplifier 14, a local oscillator 16 tuned to a prescribed carrier frequency, mixers 18a and 18b, and lowpass channel filters 20a and 20b. As recognized in the art, I and Q channel signals are generated based on modulating a signal by a first carrier and a second carrier phase-shifted by π/2 (i.e., 90 degrees), respectively. The received signal is supplied to the mixers 18a and 18b. The mixer 18a outputs a first demodulated signal that includes the I component and a first carrier component (e.g., a sine wave); the mixer 18b, having received a phase-shifted carrier signal from the phase shifter 22, outputs a second demodulated signal that includes the Q component and a second carrier component (e.g., a cosine wave). The low pass filters 20a and 20b remove the respective carrier components and output the I and Q components, respectively.
A particular concern involves IQ imbalances in direct conversion receiver architecture. In particular, the I and Q components in theory should have the same respective amplitude and phase. However, the phase and amplitude of the I and Q components output by the direct conversion receiver 10 are not the same; hence, I/Q imbalance compensation is necessary to avoid deterioration of the signal to noise ratio which may prevent decoding of the received packet.
Amplitude and phase imbalances can be corrected in either in the time domain or the frequency domain in an OFDM system. The effect of the amplitude/phase imbalance is expressed by the following relation quantitatively.
Assuming for the following analysis that there is no frequency offset and channel characteristics have a flat frequency response, Let:
ak be the Desired Symbol at sub-carrier k in an OFDM signal, and
âk be the Received Symbol at carrier k after imbalance (phase θ, amplitude α)
Then
            a      ^        k    =                    (                              a            k                    +                                    α              2                        ⁢                          a                              -                k                                                    )            ⁢              cos        ⁡                  (                      θ            2                    )                      +                  j        ⁡                  (                                                    α                2                            ⁢                              a                k                                      -                          a                              -                k                                              )                    ⁢              sin        ⁡                  (                      θ            2                    )                    
Where the “Received symbol” is the combination of the desired symbol and the symbol at the image sub-carrier.
However, the above analysis is only valid and accurate if the frequency offset is minimal between the receiver and the transmitter and the channel charactersitic between them is a flat frequency response. In a real world, the effects of channel and clock offset dominate and the channel estimation algorithm would not be able to distinguish between the effects of imbalance, channel, frequency offsets. Therefore the estimation of phase/amplitude imbalance from a signal received from an another station will yield inaccurate results.